1. Field of the Invention
The present invention relates to a travel sensor and, more particularly, to a travel sensor for measuring a penetration depth of a body in a measuring coil through which an alternating current passes by measuring the damping of the alternating current.
2. Prior Art
From DE-OS 31 09 930.0 a travel sensor is known, in which the bar core with the exploring coil is inserted into a copper tubing attached inside a pipe. The frequency of the alternating current flowing through the coil is timed in such a way, that the eddy currents only develop in the copper surface. But the so-called eddy current principle used here has a low temperature dependency. However, at differing temperatures a temperature drift occurs which is a function of the depth of penetration of the insertion body into the exploring coil. In an electrical evaluation circuit this temperature drift can only be compensated with difficulty and requires a lot of time.
In the inductive travel sensor known from DE-OS 40 20 369.7 an insertion body is also moved in a measuring coil. In this case the insertion body consists of a layer of ferromagnetic and a layer of non-ferromagnetic material with good electrical conductivity properties. However, in practice it is very difficult to apply these two layers and to synchronize the two in order to avoid a temperature drift. For this reason the manufacture of this type of travel sensor is very time consuming, complicated and relatively expensive.